According to Sefer Yetzirah, Monday is ruled by the planet Mars, as opposed to the moon as one finds in traditional western astrology. The Hebrew archetype for the planet Mars is Yitzchak, and the Celtic archetype is Tiw.
Tiw is the archetypal power which governs War, Justice and The Law. Transliterated into Hebrew letters, Tiw may be spelled tet-yud-vav טיו. These letters in reverse vision are ezer knegdo to the letters vav-yud-tet ויט, meaning "turned away", "stretched out", "turned in", "extended" and "and pitched his tent". The word ויט is found in Bereshit 12:8 -
From there he relocated to the mountain east of Beth-el and pitched his tent, with Beth-el on the west and Ai on the east; and he built there an alter to Hashem and invoked Hashem by Name.
ויעתק שמם ההרה, מקדם לבית-אל -- ויט אהלה; בית-אל מים, והעי מקדם, ויבן-שם מזבח ליהוה, ליקרא בשם יהוה
ויט is also found in Bereshit 26:25 -
He built an alter there, invoked Hashem by Name, and there he pitched his tent; there Yitzchak's servants dug a well.
ויבן שם מזבח, ויקרא בשם יהוה, ויט-שם, אהלו; ויכרו-שם עבדי-יצחק, באר
Interestingly, both times in Torah, the reverse vision of Tiw is concerned with invoking Hashem by Name. In the second case, the letters ויט are themselves explicitly connected to the word "shem" שם (Name).